1. Field of the Invention
The present invention relates to a method for producing an optical waveguide, and, more particularly, a method for producing a flexible polymer optical waveguide and a laminated polymer optical waveguide, both equipped with a plurality of alignment marks.
2. Description of the Related Art
As methods for producing a polymer waveguide, conventionally there is produced (1) a process of saturating a film with a monomer, selectively exposing a core part to light in order to change a refractive index, and laminating films (selective polymerizing process); (2) a process of coating a core layer and a cladding layer, and forming a cladding part using a reactive ion etching (RIE process); (3) a process of using a photolithography method of performing exposure to light and development using an ultraviolet ray-curable resin in which a photosensitive material is added to a polymer material (direct exposing process); (4) a process of utilizing injection molding; and (5) a process of coating a core layer and a cladding layer, and exposing a core part to light in order to change a refractive index of the core part (photobleaching process). These are several representative examples of wavelength producing methods.
However, the selective polymerizing process of (1) has a problem in film laminatio, and processes of (2) and (3) are expensive due to the use of photolithography methods, and process (4) is problematic in the precision of the diameter of the resulting core. In addition, a process of (5) has a problem that a sufficient difference in refractive indices between a core layer and a cladding layer can not be obtained.
Currently, of the above process, only (2) and (3), are both practical and excellent in performance, however, as mentioned above, they are problematic in that they are costly. Further, none of the processes (1) to (5) are suitable for forming a polymer waveguide on a flexible plastic substrate having a large area.
In addition, a method for producing a polymer optical waveguide is known where by filling a polymer precursor material for a core into capillary grooves formed on a patterned substrate (cladding), and thereafter curing the material to form a core layer, and laminating a planar substrate (cladding) thereon. However, this process has a drawback in that the polymer precursor material is filled thinly not only in the capillary grooves but also entirely between the patterned substrate and the planar substrate, causing the formation of a thin layer having the same composition as that of the core layer and, as a result, light leaks thorough this thin layer.
In order to solve this problem, David Hart proposed a method for producing a polymer optical waveguide by claimping a capillary groove-patterned substrate to a planar substrate with a clamping jig, sealing their contacting parts with a resin, and reducing the pressure to fill a monomer (diallyl isophthalate) solution into the capillary (Japanese Patent No. 3151364). This process is for reducing the viscosity of the filling material by using, as a core-forming resin material, a monomer instead of a polymer precursor material. Further, by filling the material into the capillaries by utilizing the capillary phenomenon, parts other than the capillary grooves do not get monomer filled or applied thereto.
However, since this process uses a monomer as the core-forming material, the volume shrinking rate upon polymerization of a monomer into a polymer is considerable, and permeation loss in the polymer optical waveguide becomes problematic.
In addition, this process involves complicated steps such as claimping a patterned substrate and a planar substrate with a clamp, or additionally sealing their contacting parts with a resin. Thus, this process is not suitable for large scale production and, as a result, is not practical from the standpoint of cost reduction. Moreover, this process can not be applied to preparation of a polymer optical waveguide using, as a cladding, a film having a thickness of mm scale of or 1 mm or less.
Recently, George M. Whitesides of Harvard University has proposed, as a new technique for making a nanostructure, a soft lithography process called capillary micromolding. This is a process of making a master substrate by utilizing photolithography, transferring a nanostructure of a master substrate onto a mold of polydimethylsiloxane (PDMS) utilizing the adherability and the easy peelability of PDMS, and pouring a liquid polymer into this mold utilizing the capillary phenomenon and solidify the polymer. An article explaining the details is described in Scientific American, September 2001 (Nikkei Science, December 2001).
Or, Kim Enoch et al. of Harvard's George M. Whitesides group filed a patent application directed to a capillary micromolding method (U.S. Pat. No. 6,355,198).
However, even when the preparing process described in this patent is applied to preparation of a polymer optical waveguide, it is still problematic. This is due to the fact that forming the core part is time consuming due to its small cross-sectional area, so this method is unsuitable for large-scale production. Furthermore, the process has shortcomings in that a change in volume is caused upon polymerization of a monomer solution into a polymer, the shape of the core is changed, and permeability significantly increases.
In addition, B. Michel et al. of IBM's Zürich Laboratory have proposed a high-resolution lithography technique using PDMS, and have reported that a resolving power of dozens of nm can be obtained with this technique. There is an article explaining the details of this techniques in IBM J. REV. & DEV. VOL. 45 No. 5, September 2001.
The above PDMS soft lithography technique and capillary micromolding method are techniques which have recently attracted a lot of attention, mainly in the USA.
However, the aforementioned micromolding method can not satisfy the need to both reduce the volume shrinking rate upon curing (therefore reduce permeation loss) and reduce the viscosity of the filling liquid (monomer, etc.) for easy filling. Therefore, when reducing permeation loss is a priority, since the viscosity of the filling liquid can not be decreased below a certain limit, the filling rate is decreased and large-scale production can not be expected. In addition, the aforementioned micromolding method functions on the premise that a glass or silicon substrate is used as the substrate, and use of a flexible film substrate is not taken into consideration.
In contrast, the present inventors have proposed a method for producing a flexible polymer optical waveguide in which an optical waveguide is provided on a film substrate, at extremely low cost, in Japanese Patent Application No. 2002-187473. A polymer optical waveguide prepared by this process sustain minimal permiation loss and maintains a highly precise core shape and further, since the waveguide is flexible as a whole, it can be freely installed into various apparatuses. In order to increase the degree of circuit integration, structures in which polymer optical waveguides are laminated or laminated on an electric circuit substrate are effective. However, accurate positioning upon lamination of a flexible polymer optical waveguide is quite difficult.
hence, there is a need for an invention that can produce an optical waveguide that provides easy and accurate positioning.